Aiming to improve traffic flow onto Interstate 96 from Cascade Road, the Michigan Department of Transportation (MDOT) decided to replace an aging bridge and loop system with a $15 million diverging diamond interchange.

"The project is being driven by the condition of the existing bridge," says John Richard, MDOT Communications. "It has served us well, but it is time for a replacement. The new project will address operational issues on Cascade and improve the ramps of Interstate 96." 

About 65,000 vehicles use Cascade Road daily, with back ups occurring in the left turn lane, seeking entry onto the interstate. The existing bridge was built in 1961. The deck and substructure had deteriorated. MDOT considered a rehabilitation project but faced with traffic issues and that the existing loop ramps did not meet current standards for acceleration and deceleration, the department thought better of it.   

"In order to bring the remainder of the ramps into compliance, we'd have to do some work on the road," says Chris VanNorwick, MDOT Engineer Grand Rapids. "So we started looking at a diverging diamond interchange."

About a Diverging Diamond

With a diverging diamond, traffic crosses to the opposite side of the road, travels over a bridge and then crosses back over another bridge to return to the standard roadway configuration. It eliminates left-turn lanes and left-turn signals. Traffic exits flows left off the diamond onto the interstate. Signs are placed on either side. Two-phase signals keep traffic in check. The speed limit will be lower than on the rest of Cascade Road.

"This operates much smoother and more efficiently," VanNorwick says. "Other important benefits are safety improvements."

Currently, 57 diverging diamond interchanges exist in the United States, according to Advanced Transportation Solutions in Silver Spring, Maryland. Last year, the Federal Highway Administration produced an informational guide about the interchanges. The agency has studied the diverging diamond and reports that it offers safety benefits.

The unusual interchange configuration is popular in France. That's where Gilbert Chlewicki, President and CEO of Advanced Transportation Solutions, first experienced the diverging diamond while a student at the University of Maryland in College Park. He had designed something similar, an at-grade interchange designed to enhance traffic flow. He reported feeling happy the concept was in use but disappointed his idea for the intersection-style was not the first. He wrote a paper and presented it. Then the Federal Highway Administration began studying the concept. 

Missouri pioneered the interchange in 2009 in Springfield, and Michigan met with Missouri DOT officials to learn about the diverging diamond, discuss pros and cons, and see first hand how they are working.

"Missouri DOT is seeing a large reduction in serious and injury crashes, because of the type of interchange," VanNorwick says. "It slows traffic down and there is a reduction in conflict points." 

Missouri crash analyses indicates an average 40 percent reduction in crashes with diverging diamonds and an 80 percent reduction in crashes with injuries. Rear end crashes decreased and right angle crashes and wrong-way movements on and off the ramps were nearly eliminated.

Pedestrians cross one half of the road at a time, with all of that traffic flowing in the same direction. As a result of what the MDOT team learned in Missouri, it is adding special sidewalk markings that say "Look right" to remind pedestrians that the traffic will be coming from a different direction than they are used to it coming from. The state is building a 4-foot bike lane and making the traffic lanes wider than normal to accommodate trucks and prevent them from drifting into adjusting lanes.

TranSystems of East Lansing, Michigan, designed the interchange. The state does not currently have design standards for a diverging diamond interchange, so much of the guidance came from Missouri and federal officials.

Another challenge was educating drivers about the diverging diamond and why the design was selected. The information includes videos about the interchange, showing how traffic will flow and giving views from a "driver's perspective. The advisory speed limit will drop from 45 mph to 25 mph at the interchange.

"Because of the angles of the curves coming into the diamond, it acts naturally as a traffic calming scenario," VanNorwick explains. 

MDOT also will add an extra turn lane to Cascade in both directions. The Kent County Road Commission has a resurfacing project to the east of the MDOT interchange.  

MDOT also is installing a diverging diamond interchange on I-75 at University Drive in Auburn Hills, in the eastern part of the state. This is a design-build project, expected to finish late this year.

Constructing the Diamond

Sequencing provided another benefit of the diverging diamond design. In August of this year, Milbocker & Sons of Allegan, Michigan, started construction of the southern bridge, offline from the existing roadway with minimal effects to Cascade Road traffic. Then in 2016, traffic will be shifted to the new bridge, and Milbocker will complete construction of a second bridge adjacent to, and north of, the first bridge. 

"Other than additional staging, the processes involved in constructing this diverging diamond interchange are similar to those used in most other interchange projects," says Tom Pratt, Project Manager with Milbocker & Sons.

The bridges will have mechanically stabilized earth walls, pile supported concrete abutments, piers on spread footings and prestressed concrete beams. 

"There's nothing out of the ordinary construction-wise," Pratt says. "It's just that with this design concept, when the project is completed, traffic will be flowing on the "˜wrong side' of the road when crossing over the bridges."